Benzimidazole functionalized covalent triazine frameworks for CO2 capture

Article abstract of DOI:10.1039/c6ta05107c

A series of covalent triazine frameworks containing benzimidazole segments (CTF-BIs) were designed and prepared by dynamic trimerization under ionothermal conditions, based on the novel designed dicyano monomer of dicyano benzimidazole (DCBI). The CTF-BIs

Full text of DOI:10.1039/c6ta05107c

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Benzimidazole functionalized Covalent Triazine Frameworks for
CO2 capture
Liming Tao,a Fang Niub, Chao Wanga, Jingang Liuc, Tingmei Wanga and Qihua Wang*a
Received 00th January 20xx,
Accepted 00th January 20xx
A series of covalent triazine frameworks containing benzimidazole segments (CTF-BIs) were designed and prepared by the
dynamic trimerization under ionothermal conditions, based on the novel designed dicyano monomer of dicyano
benzimidazole (DCBI). The CTF-BIs were all amorphous and exhibited BET surface areas as high as up to 1549 m2 g-1. The
effect of reaction conditions including ratio of catalyst and temperature on the pore properties was discussed. Meanwhile,
the capacity of CO2 adsorption for all the CTF-BIs was studied and the CO2 uptake capacity was as high as up to 21.68wt%
at 273 K and 1.10 bar for CTF-BI-10. The CTF-BIs exhibited high selectivity of CO2 over N2 in the range of 31.3~88.5 at 273 K,
and 29.0~102.7 at 303 K, respectively.
DOI: 10.1039/x0xx00000x
www.rsc.org/
porous organic polymers known to date.9 Till now, most of the
reported BILPs were synthezed by the solution
polycondensation reactions between aryl-o-diamine and aryl-
Introduction
Recently, due to the urgent need for strategies to reduce
global atmospheric concentrations of greenhouse gases, the
carbon capture and storage (CCS) has attracted significant
interest. According to the International Energy Agency (IEA),
appropriate capture and storage of post-combusted CO2 has
aldehydes. The aryl-o-diamine monomers are easily oxidized
and the stoichiometric should be strictly controlled. The
benzimidazole groups are all generated during the
polymerizations. While porous polymers derived from
polymerization of benzimidazole containing monomers have
been rarely researched.
the potential of decreasing the emissions up to 20%.1,
2
Tremendous efforts have been made to the design and
synthesis of microporous organic polymers (MOPs) for CO2
capture or selective adsorption because of their high
physicochemical stability and excellent adsorption capacity.3-7
Therefore, various CO2-philic moieties have been incorporated
into MOPs to enhance the interactions between the MOPs’
surfaces and CO2 molecules. The CO2-philic moieties usually
contain heteroatoms (e.g. N, P) which have Lewis basic
properties. They can simultaneously enhance CO2 capture
capacity and selectivity through Lewis acid−base interacꢀons.8-
11 Various kinds of nitrogen containing groups, including azos12-
On the other hand, as a kind of nitrogen rich materials,
covalent triazine-based frameworks (CTFs) were usually
prepared by the ionothermal polymerization at high
temperatures, where ZnCl2 was used as both catalyst and
solvents.25-28 The proper monomers were aromatic compounds
with multiple cyanos. Under high temperature and the
catalysis of molten ZnCl2, the trimerization of cyano groups
occurred. The molar ratio of monomer to ZnCl2, the reaction
temperature and time were the main factors which had
important effect on the final results. Stoichiometric was not
necessary because there was only one kind of monomer in the
reaction system. Although with high nitrogen loadings in CTFs,
the CO2 adsorption abilities were not so high. Therefore,
fluorine atoms29 and fluorene groups30 were introduced into
CTFs respectively, which enhanced the CO2 uptakes obviously.
However, CTFs containing benzimidazole groups synthezed
taking advantage of the ionothermal procedure mentioned
above have not been reported so far.
14
troger’s base,15, 16 benzimidazole,17-20 aminos,21 carbazole,22,
23
and porphyrins,24 have been incorporated into MOPs.
Among them, benzimidazole linked polymers (BILPs) recently
reported by H. M. El-Kaderi and co-workers showed promising
performances for selectively CO2 capture. These CO2 uptakes
are as high as 24wt% (BILP-4) among the highest values in all
In the present work, we prepared benzimidazole-linked CTFs
(CTF-BIs) for the first time by use of benzimidazole containing
monomers through the ionothermal polymerization. We first
designed and synthesized a novel benzimidazole derived
dicyano monomer, 2-(4-cyanophenyl)-1H-benzo[d]imidazole-5-
carbonitrile (DCBI), and then DCBI was polymerized to CTF-BIs
under different conditions including molar ratio to the catalyst
of ZnCl2 and the reaction temperatures. We expected that the
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CO2-philic benzimidazoles would enhance the binding affinity were recorded on a Netzsch thermal analysis system (STA
between the CTFs and CO2, and thus increased the CO2 449C), in nitrogen, at a heating rate of 10 °C/min. Elemental
adsorption of CTFs. The synthesis and characterization of the analysis was obtained with Vario EL analyser. FESEM images
novel monomer DCBI, the synthesis and properties of the CTF- were obtained with
a field emission scanning electron
BIs, the effect of reaction conditions on the surface areas, pore microscope (JSM-5600LV, JEOL, Japan). The powder samples
volumes and distribution, CO2 adsorption ability and the were coated with gold by sputtering prior to observation. HR-
selectivity of CO2 over N2, had been studied in detail.
TEM images were obtained with A JEM-2010 transmission
electron microscopy, operating at an accelerating voltage of
200 KV. Nitrogen sorption experiments and micropore analysis
were conducted at 77.3 K using Micromeritics ASAP 2020
HD88. Before sorption measurements, the samples were
degassed in vacuum overnight at 150 °C. The surface areas
were calculated from multipoint BET plot, and the pore
volume was determined by nonlocal density functional theory
(NLDFT). Carbon dioxide and nitrogen sorption isotherms at
273 K and 303 K were all obtained with a Micromeritics ASAP
2020 HD88 analyzer at the required temperature. The
temperature during adsorption and desorption was kept
Experimental
Materials
All
chemicals
including
3,4-diaminobenzonitrile,
4-
formylbenzonitrile, ceric ammonium nitrate (CAN), zinc
chloride (ZnCl2, anhydrous, 98%) and trifluorosulfonic acid
(TFSA) were all purchased from J&K Scientific Ltd. (Beijing) and
used without further purification. DCBI was synthesized in our
laboratory according to the literature.31 ZnCl2 was stored in a
glove box and measured there.
constant using
a
circulator. Before the adsorption
Characterization
measurements, the samples were activated in situ by
increasing the temperature at a heating rate of 10 °C/min up
to 150 °C under vacuum and the temperature and vacuum
were maintained for 5 hours before taking the sorption.
Fourier transform infrared (FT-IR) spectra were recorded on a
Perkin-Elmer 782 Fourier transform spectrophotometer. The
powder wide-angle X-ray diffraction (PXRD) was conducted on
a Rigaku D/max-2500 X-ray diffractometer with Cu/K-α1
radiation, operated at 40 kV and 200 mA. Thermogravimetric
analysis (TGA) and Differential Scanning Calorimetry (DSC)
Table 1 Overview of the reaction conditions.
Polymer code
CTF-BI-1
CTF-BI-2
CTF-BI-3
CTF-BI-4
CTF-BI-5
CTF-BI-6
CTF-BI-7
CTF-BI-8
CTF-BI-9
CTF-BI-10
CTF-BI-11
DCBI (mmol)
ZnCl2 (mmol)
0.50
1.0
Molar ratio of DCBI/ZnCl2
Reaction conditions
400 °C/40 hrs
400 °C/40 hrs
400 °C/40 hrs
400 °C/40 hrs
400 °C/40 hrs
400 °C/40 hrs
400 °C/40 hrs
350 °C/40 hrs
450 °C/40 hrs
500 °C/40 hrs
550 °C/40 hrs
Yield (%)
92
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2:1
1:1
90
2.0
1:2
90
5.0
1:5
94
10.0
20.0
50.0
5.0
1:10
1:20
1:50
1:5
94
94
94
84
5.0
1:5
81
5.0
1:5
81
5.0
1:5
76
Synthesis
of
2-(4-cyanophenyl)-1H-benzo[d]imidazole-5- 2H); 8.29 (d, 1H); 8.09~8.07 (d, 2H); 7.90~7.88, (tetra, 1H);
carbonitrile (DCBI)
7.68~7.66 (d, 1H).
Synthesis of the CTF-BIs
In a 250 mL three-necked flask with a magnetic stir and N2
inlet, 2.623 g (20 mmol) 4-formylbenzonitrile and 2.663 g (20 The CTF-BIs were synthesized according to
a reported
mmol) 3,4-diaminobenzonitrile were dissolved in 70 mL literature.25 The synthesis of CTF-BI-4 was given as an example
acetonitrile. Then, H2O2 (30%, 80 mmol, 8.0 mL) and (Scheme 1). The DCBI monomer (244.3 mg, 1.0 mmol) and
NH4Ce(NO3)6 (2.0 mmol, 1.096 g) were added and the mixture ZnCl2 (682 mg, 5.0 mmol) were first mixed well in agate mortar
was stirred at room temperature for 8 hours. After completion in glove box and then transferred into a pyrex ampoule (3 × 18
of the reaction, the reaction mixture was poured into ice- cm, about 25 mL). The ampoules were evacuated to vacuum,
water (400 mL). The pure solid product was filtered, washed sealed and heated to 400 °C and kept there for 40 hrs. The
with ice-water, and subsequently dried. The crude product was ampoule was then cooled down to room temperature and
recrystallized in EtOH to give fine powder in light red-brown. opened carefully. The black reaction mixture was subsequently
(Yield, 4.32 g, 88%; mp: 322.9 °C by DSC in N2, 10 °C/min). FT- grounded in an agate mortar and then washed thoroughly with
1
IR (KBr pellets, cm-1): 3276, 2232, 1609, 1421, 1320, 860. H- diluted HCl to remove most of the ZnCl2. Further purification
NMR (400 MHz, DMSO-d6, δ, ppm): 13.75 (s, 1H); 8.38~8.36 (d, including extracting in Soxhlet extractor by deioned water and
THF for 24 hrs each and drying in vacuum at 150 °C. The yield
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was about 94%. The other CTF-BIs were synthesized in a clearly according to the integral values. The DSC curve of DCBI
similar procedure, while the reaction conditions, including the (Fig. S3†) showed the melting point at about 322.9 °C. In the
molar ratio of monomer to the catalyst and reaction TGA curve (Fig. S4†), the first/main weight loss started at
temperatures, were set according to Table 1. For comparison, about 350 °C indicated the decomposition of benzimidazole
the polymerization was also carried in 1,2-dichloroethane rings.
solution by using TFSA as catalyst, according to a reported Synthesis and characterization of the CTF-BIs
method.32
The CTF-BIs were first synthesized by the procedure of
ionothermal polymerization, which was first reported by A.
Thomas et al.25 In order to find the relationship between the
reaction conditions and the porosity properties, a set of
reaction conditions which were different in reaction
temperatures and molar ratios was listed in Table 1. The molar
ratio of DCBI to ZnCl2 was set as 2:1, 1:1, 1:2, 1:5, 1:10, 1:20,
and 1:50, respectively. The reaction temperature was set as
350 °C, 400 °C, 450 °C, 500 °C and 550 °C. All the reaction time
was set as 40 hours. In all cases, after polymerization, the light
brown powdery reactants turned black dust in the sealed tube.
Scheme 1 Schematic representation of the synthesis of CTF-BIs
Results and discussion
Synthesis and characterization of DCBI
The monomer DCBI was synthesized by the oxidative coupling For CTF-BI-1 to CTF-BI-8, there were no positive pressures
of 3,4-diaminobenzonitrile and 4-formylbenzonitrile catalyzed when the sealed tube was open. In comparison, for CTF-BI-9,
by CAN at room temperature, with a good yield in a short CTF-BI-10 and CTF-BI-11 reacted at 450 °C, 500 °C, and 550 °C,
reaction time (Scheme 1). The molecular structure was there was obviously positive pressure, which should be paid
1
confirmed by FT-IR spectrometry and H NMR spectrum. Fig. more attention. All black dusts were rounded in agate mortar
S1† compared the FT-IR spectra of DCBI and the corresponding to get fine powders. The black powder were all immersed in
reactants. For 3,4-diaminobenzonitrile, the broad and strong
absorption bands between 3100 cm-1 and 3450 cm-1
disappeared totally after coupling, indicating the amino had
been consumed. At the same time, for 4-formylbenzonitrile,
after coupling, the strong band of aldehyde at 1707 cm-1 also
disappeared. Accordingly, the characteristic bands of
benzimidazole rings at 3276 cm-1 appeared obviously, which
can be attributed to the –NH– stretching vibration. The new
bands appeared at 1609 cm-1 (C=N), and 1495 and 1435 cm-1
can be assigned to skeleton vibration of the benzimidazole
ring.9 Meanwhile, the characteristic absorption bands of –CN
groups can also be observed clearly at 2232 cm-1. In the 1H
NMR spectrum shown in Fig. S2†, all protons could be assigned
deioned water and washed by diluted HCl and extracted by hot
water, to remove most of the residual ZnCl2. The final yields
were in the range of 76%-94%, some of which were a little
lower than the reported results due to the high reaction
temperature. As the reaction temperature raised, the final
yields decreased gradually maybe due to the partially
decomposition of the triazine rings and/or the benzimidazole
groups. For comparison, the polymerization catalyzed by TFSA
in solution was also carried out at room temperature (CTF-BI-
S). The formation of CTF-BIs was characterized by FT-IR, solid-
state 13C NMR spectra, PXRD, and thermogravimetric analysis
(TGA).
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Fig. 1 (a) and (b) FT-IR spectra of the monomer DCBI and CTF-BI-4 and CTF-BI-S obtained by KBr pellets, in the range of 4000 cm-1~400 cm-1 and 2000 cm-1~400 cm-1, respectively;
(c) Solid-state 13C NMR spectra of DCBI, CTF-BI-4, and CTF-BI-11 (100 MHz); (d) PXRD patterns of CTF-BI-4 and CTF-BI-11; (e) FESEM image of CTF-BI-4; (f) HR-TEM of CTF-BI-4.
Representative FT-IR spectra of a CTF (CTF-BI-4, CTF-BI-S) and rings at higher temperatures, which in well accordance with
DCBI were shown in Fig. 1a and 1b. After polymerization, the the FT-IR results as mentioned above.
cyano groups at about 2232 cm-1 were almost totally The CTF-BIs were also characterized by thermogravimetric
disappeared for both CTF-BI-4 and CTF-BI-S, indicating the high analysis (TGA) and powder X-ray diffraction (PXRD). As shown
degree of trimerization. The characterized peaks of triazine in Fig. S7†, CTF-BI-11 exhibited remarkable thermal stability up
rings were not very strong, which were located at 1517 cm-1 to 600 °C in N2. The initial weight loss before 200 °C can be
and 1353 cm-1,25 maybe overlapped by nearby peaks. The attributed to the absorbed water. The weight loss after 400 °C
other FT-IR spectra were all shown in Fig. S5† and S6†. For can be attributed to the degradation of organic frameworks.
CTF-BI-1 and CTF-BI-2, the cyano groups can still be observed Therefore, they have similar thermal stability compared with
9
clearly, indicating a low degree of trimerization. For the CTF- the reported BILPs.8, CTF-BI-11 showed high char yields
BIs obtained from higher reaction temperatures, the peaks of higher than 70% at 800 °C. Due to the nature of their rigid
triazine rings were even weaker, maybe due to the partially structures, there was no observable glass transition
decomposition or fragmentation.25,
33
In the solid-state 13C temperature within the range of 30-500 °C (Fig. S8†), just like
NMR spectra shown in Fig. 1c, the chemical signals located at most of reported MOPs. The PXRD patterns of CTF-BI-4 and
105.4 and 111.6 ppm should be assigned to the asymmetric CTF-BI-11 shown in Fig. 1d indicated no long-range structures.
nitrile carbons. After polymerization they almost disappeared They were amorphous with broad diffraction peaks at about
34
totally, indicating the high reaction extent.30,
The signal 22.8 o, which was in well accordance with the reported
located at 150.1 ppm in DCBI should be assigned to NC(Ph)N in results.11 The morphology was also characterized by FESEM as
benzimidazole units.30, 35 After polymerization at 400 °C, these shown in Fig. 1e and S9†, which revealed aggregated particles
signals can be still seen in CTF-BI-4 (152.6 ppm). However, for of variable size of 1~10 μm. In the HR-TEM images shown in Fig.
CTF-BI-11 obtained at 550 °C, the intensity attenuated 1f and S10†, the alternating areas of light and dark contrast
obviously maybe due to the decomposition of benzimidazole revealed their disordered porous structural natures.
Table 2 Porosity data for the CTF-BIs from N2 isotherms collected at 77.3 K
Polymer code
CTF-BI-1(g)
CTF-BI-2(g)
CTF-BI-3
SABET [m2 g-1](a)
SALang [m2 g-1](b)
SAmicro [m2 g-1](c)
SAext [m2 g-1](d)
V0.1 [m3 g-1](e)
0
Vtot [m3 g-1](f)
0
V0.1/Vtot
-
0
0
0
0
0
0
0
0
0
0
-
677
1025
836
759
642
55
992
1505
1236
1117
938
81
531
762
647
470
358
38
146
263
190
289
284
17
0.33
0.50
0.41
0.36
0.30
0.026
0.44
0.51
0.64
0.0007
0.36
0.58
0.46
0.45
0.38
0.034
0.49
0.59
0.88
0.0079
0.92
0.86
0.89
0.80
0.79
0.76
0.89
0.86
0.73
0.09
CTF-BI-4
CTF-BI-5
CTF-BI-6
CTF-BI-7
CTF-BI-8
CTF-BI-9
885
1099
1549
2
1308
1641
2320
2.4
699
669
335
0.4
186
431
1214
1
CTF-BI-10
CTF-BI-11
CTF-BI-S
(a) BET surface area calculated over the pressure range 0.05-0.30 P/P0 at 77.3 K; (b) Langmuir specific surface area calculated from the nitrogen adsorption isotherm by
(c)
(d)
application of the Langmuir equation; Micropore surface area calculated from the nitrogen adsorption isotherm using the t-plot method; The external surface
(e)
(f)
areas calculated using the t-plot method based on the Halsey thickness equation; V0.1, pore volume at P/P0 = 0.1 at 77.3 K; Vtot, total pore volume calculated at
P/P0 = 0.99 at 77.3 K; (g) Not detected any reasonable data.
Porosity properties and gas uptake capacities of the CTF-BIs
surface area of CTF-BIs was varied from 938 to 2320 m2 g-1.
None of them showed remarkable hysteresis at the low
pressure region in the N2 isotherms. Therefore, the dominating
pores were micropores, which were in well accordance to the
pore size distribution as shown in Fig 2b, 2e, S13†, and S15†.
For CTF-BI-11, there was only a little mesopores (2.29 nm and
2.52 nm, Fig 2e), which can be ascribed to the partially
decomposition of the polymeric frameworks under high
temperature. For CTF-BI-S obtained in the solution
polymerization, although the reaction extent was high
according to the FT-IR spectra (Fig. 1a), the surface area was
The porous character of the CTF-BIs was studied by nitrogen
adsorption-desorption experiments at 77.3 K. All samples were
degassed at 150 °C overnight under vacuum prior to the
measurement. As shown in Fig. 2a, 2d, S11†, S12†, and S14†,
except CTF-BI-1, CTF-BI-2, CTF-BI-8, and CTF-BI-S, all the
isotherms show a steep increase in adsorbed volume at low
relative pressure, suggesting that these CTF-BIs were
microporous materials. All the seven CTF-BIs showed type I N2
sorption isotherms with Brunauer-Emmett-Teller (BET) surface
areas ranging from 642 to 1549 m2 g-1 (Table 2). The Langmuir
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very small (Fig. S11† and Table 2). The nonporous nature nonporous, which probably because the low mass of ZnCl2 was
indicated the formation of oligomers with low molecular not sufficient in such a reaction mixture to fully dissolve the
weights.
fluffy and powdery monomers and the produced oligomers.25
It was found that the physical properties such as BET surface There was still a large part of nitriles residue in the resultant
area, Langmuir surface area, micropore surface area, mixture judging from FT-IR spectra (Fig. S5†). When the ratios
micropore volume, and bulk density could be affected by of DCBI to ZnCl2 increased from 1:2 to 1:50, the BET surface
adjusting the synthetic conditions.26,
28
Therefore, we areas increased first and then decreased gradually, as shown in
investigated the influence of the amount of ZnCl2, as well as Fig 2c, which was in good agreement with that of dicyano
reaction temperature on the properties of the CTFs and we set bisphenyl (DCBP) under similar conditions.36 For CTF-BI-4,
a series of reaction conditions (Table 1). First, the influence of where the ratio of DCBI to ZnCl2 was 1:5, the BET surface area
the DCBI/ZnCl2 ratio was investigated under standard was as high as 1025 m2g-1, which was a little lower than that of
conditions at 400 °C. The amount of ZnCl2 used had obvious CTF-1 derived from 1,4-dicyanobenzene (DCB) under similar
effect on the pore properties, because it was both catalyst and conditions (1123 m2g-1). Further increase of ZnCl2 amount lead
solvent in the reaction. When the amount was too little, i.e. to decrease in BET surface areas may be due to the dilutive
the ratios of DCBI to ZnCl2 were 2:1 and 1:1, the BET surface effect of excess amount of solvents.
areas were too small to be detected and the materials were
Fig. 2 Porosity properties of CTF-BIs. (a) Nitrogen adsorption and desorption isotherms at 77.3 K of CTF-BI-4; (b) Pore size distributions (PSD) for CTF-BI-4 calculated by the NLDFT
method; (c) The influence of molar ratio (DCBI to ZnCl2) on the surface areas; (d) Nitrogen adsorption and desorption isotherms at 77.3 K of CTF-BI-11; (e) Pore size distributions
(PSD) for CTF-BI-11 calculated by the NLDFT method; (f) The influence of reaction temperature on the surface areas.
As previously found for the polymerization of DCB,25 higher lower than that of CTF-0 derived from 1,3,5-tricyanobenzen
reaction temperatures generated mesoporosity in addition to (TCB) under similar conditions (2011 m2g-1).28 It can be partially
microporosity. Thus a series of temperatures from 350 °C to ascribed to the relatively longer strut building block compared
550 °C was investigated. When reaction temperatures with TCB.37 Furthermore, higher temperatures lead to some
increased, the BET surface areas increased gradually, as shown side reactions such as decompositions and ring fragmentations,
in Fig. 2f. For CTF-BI-11 obtained at 550 °C, the BET surface as evidenced by FT-IR and solid-state 13C NMR analysis. In the
area was as high as 1549 m2g-1, much higher than that of CTF- present system, both the triazine rings and the benzimidazole
BI-4, which can be ascribed to the appearance of mesoporosity rings could decompose under higher temperatures, leading to
in addition to microporosity. However, this value was much nitrogen doped porous carbons.
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Fig. 3 (a) CO2 adsorption isotherms at 273 K of CTF-BIs; (b) CO2 adsorption isotherms at 303 K of CTF-BIs; (c) The ratios of four N-configurations (N(x)) in CTF-BIs; (d) Heat of
adsorption for CTF-BI-4 and CTF-BI-11.
Once the porosity of CTF-BIs had been established, we CTF-0 (18.57wt%) derived from TCB,28 CTF-1 (10.87wt%–
considered their applications in gas storage. There were two 16.81wt%) based on DCB,29 CTF-P1M–P6M (4.14wt%–
kinds of CO2-philic groups in the CTF-BIs obtained herein, e.g., 18.48wt%) prepared by microwave method,32 MCTF300–500
the benzimidazole rings which contained secondary amine and (9.90wt%–13.90wt%) containing metalloporphyrin segments,38
the triazine groups. Therefore, it can be predicted that the PCTF-1–7 (8.10wt%–14.21wt%) derived from tetranitrile
CTF-BIs would have high CO2 adsorption ability. Generally containing tetraphenyl ethylene and adamantane segments,39,
speaking, with increasing specific surface areas and micropore 40 and TPI-1–7 (2.99wt%–10.78wt%) containing imide groups,41
volumes, the adsorption capacity for CO2 increases. We tested under similar conditions. This can be ascribed to the
the CO2 adsorption properties at 273 K and a pressure up to introduction of CO2-philic benzimidazole rings into the
1.10 bar. The adsorption data at 1.10 bar were summarized in frameworks. Even compared with other kinds of porous
Table 3, from which we can see that all CTF-BIs showed organic polymers such as covalent organic frameworks (COFs:
moderate to high capacity of CO2 adsorption, consistent with 7.92wt%–16.98wt%),42 microporous polyimides (MPIs:
our expecting. The CO2 adsorptions for all CTF-BIs ranged from 9.90wt%–16.76wt%),43
and
hyper-crosslinked
organic
11.05wt% to 21.68wt% at 273 K and 1.10 bar. For CTF-BIs polymers (HCPs; 8.46wt%–17.25wt%),44-47 CTF-BI-4 and CTF-BI-
prepared at 400 °C, the CO2 adsorption amount at 1.10 bar 11 showed high advantage not only for the adsorption ability
was in the order of CTF-BI-6 < CTF-BI-5 < CTF-BI-3 < CTF-BI-4, but also the preparation procedure which can be adapted to
the same tendency with their BET surface areas. CTF-BI-4 and large scale. However, compared with benzimidazole-linked
CTF-BI-11 had the highest CO2 adsorption amount at 1.10 bar polymers (BILPs; 12.80wt%–23.50wt%), the present adsorption
(about 21wt%). The adsorption amount was in the range of the values were a little lower than the highest one (BILP-4).9 The
highest values observed for CTFs under similar conditions. In main reason would be ascribed to the relatively high reaction
fact, besides the recently reported FCTF-1 (20.55wt%– temperatures which lead to partially decomposition and
24.33wt%),29 CTF-BI-4 and CTF-BI-11 show higher CO2 fragmentations of benzimidazole rings.
adsorption than the reported values for all other CTFs such as
6 | J. Name., 2012, 00, 1-3
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Table 3 CO2 and N2 capture capacities, CO2/N2 selectivities for the CTF-BIs
(c)
Polymer Code (a)
SBET(m2g-1)
CO2 adsorption (cm3g-1)
N2 adsorption (cm3g-1)
CO2/N2
273 K (b)
303 K
49.7
61.1
60.1
44.2
37.3
52.9
57.8
58.9
273 K
1.4
7.1
6.9
6.3
5.3
5.4
9.0
9.6
303 K
1.3
4.0
2.7
3.1
2.3
3.1
4.6
5.0
273 K
303 K
52.4
102.7
40.2
41.0
33.6
29.0
39.1
34.9
CTF-BI-3
CTF-BI-4
CTF-BI-5
CTF-BI-6
CTF-BI-7
CTF-BI-9
CTF-BI-10
CTF-BI-11
676
1025
836
77.5
88.5
44.0
35.6
34.5
39.7
67.4
31.3
34.3
108.7
100.6
75.2
759
642
56.3
885
96.1
1099
1549
99.9
110.5
(a) The CO2 adsorptions of CTF-BI-1, CTF-BI-2, and CTF-BI-8 were not measured due to the very low BET surface areas; (b) CO2 adsorptions of CTF-BIs at 273 K and 1.10
bar; (c)Selectivity estimated using the ratios of the Henry law constant calculated from the initial slopes of the single-component gas adsorption isotherms at low
pressure coverage (< 0.10 bar).
The reaction temperature played an important role to the CO2 results are plotted as a function of synthesis temperature in
adsorption ability. For CTF-BI-4 (400 °C), CTF-BI-9 (450 °C), CTF- Fig. 3c. In general, the ratio of N(2) tend to decrease when the
BI-10 (500 °C) and CTF-BI-11 (550 °C), higher reaction synthesis temperature increases; while the ratios of N(3) and
temperature lead to higher BET surface areas, but the CO2 N(4) increase with increasing synthesis temperature. The high
adsorption ability did not show
relationship. CTF-BI-10 had a higher BET surface area than that to arise from strong interactions of the polarizable CO2
of CTF-BI-4 (1099 m2 g-1 vs 1025 m2 g-1), but had a lower CO2 molecules
through hydrogen bonding and/or
a positive proportional CO2 uptakes by benzimidazole containing polymers were found
adsorption amount (19.61wt% vs 21.34wt%). Furthermore, the dipole−quadrupole interacꢀons that uꢀlize the protonated and
BET surface area of CTF-BI-11 was much higher than that of proton-free nitrogen sites of imidazole rings, respectively.9, 17
CTF-BI-4 (1549 m2 g-1 vs 1025 m2 g-1), but the CO2 adsorption The interactions between CO2 and quaternary N and N-oxide
amount was nearly in the same level with CTF-BI-4 (21.68wt% were much weaker than those of pyridinic and pyrrolic N.
vs 21.34wt%). The main reason should be again ascribed to the Although CTF-BI-11 had a much higher BET surface area, the
partially decomposition and fragmentations of benzimidazole content of quaternary N and N-oxide were also much higher
rings, as mentioned above. Table S1 showed the elemental than that of CTF-BI-4 (31.3% vs 5.0%, Fig. S16†), thus leading to
analysis results of CTF-BI-8 (350 °C), CTF-BI-4 (400 °C), CTF-BI-9 similar CO2 uptakes.
(450 °C), CTF-BI-10 (500 °C), CTF-BI-11 (550 °C). As the reaction To determine the binding affinity of CTF-BIs for CO2, we
temperature increased from 350 °C to 550 °C, the loadings of calculated the isosteric heat of adsorption (Qst) for CO2 using
N decreased gradually. It should be an evidence of the ring the Clausius–Clapeyron equation, as shown in Fig 3d and S17†.
fragmentation. X-ray photoelectron spectroscopy (XPS) The Qst values of CTF-BIs were in the range of 31.7 kJ mol-1 and
analyses of CTF-BIs were performed in order to further explore 34.3 kJ mol-1, which were well below the values expected for a
the origin of the effect of reaction temperatures on the CO2 chemisorption process (> 40 kJ mol-1). Therefore, the high CO2
adsorption ability. The N1s XPS results of DCBI and CTF-BIs (Fig. affinity of the CTF-BIs was mainly attributed to the inherent
S16†) reveal that about three to four distinct N configurations microporosity and the enhanced dipole–quadrupole
exist in the skeletons of CTF-BIs: pyridinic N (397.98–398.40 eV, interactions between the quadrupole moment of the CO2
N(1)), pyrrolic
N (399.60–399.99 eV, N(2)), quaternary molecules and the nitrogen-rich polar binding sites, both from
N(400.86–401.19 eV, N(3)), and N-oxide (402.65–403.94 eV, benzimidazole and triazine rings.
N(4)),27, 48-50 For CTF-BI-8 (350 °C), CTF-BI-4 (400 °C), and CTF- In order to investigate the gas adsorption selectivity of CTF-BIs, CO2
BI-9 (450 °C), there were three distinct N configurations while and N2 sorption properties were measured by volumetric methods
for CTF-BI-10 (500 °C) and CTF-BI-11 (550 °C), there were four, at the same conditions of 273 K and 303 K, respectively, and the
also indicated the decomposition or fragmentation reactions selectivity was estimated according to a reported method,12 in
may occur under the high temperature conditions. The atomic which the ratios of the Henry law constants calculated from the
ratios of the four N-configurations in CTF-BIs were further initial slopes of the single-component gas adsorption isotherms at
evaluated from the deconvolution of N1s peaks, and the low pressure coverage (0~100 mbar) were used (Fig. S18† to Fig.
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S26†). For all CTF-BIs, the calculated CO2/N2 adsorption selectivities 9. M. G. Rabbani and H. M. El-Kaderi, Chem. Mater., 2012, 24
were in the range of 31.3~88.5 at 273 K, and 29.0~102.7 at 303 K, 1511-1517.
respectively (Table and Fig. S26 ). The selectivities were 10. P. Arab, M. G. Rabbani, A. K. Sekizkardes, T. İslamoğlu and H.
comparable to or better than the reported fluorinated CTF (TPC-1, M. El-Kaderi, Chem. Mater., 2014, 26, 1385-1392.
selectivity of CO2/N2 at 273 K: 38),51 Cz-POFs (selectivity of CO2/N2 11. B. Ashourirad, A. K. Sekizkardes, S. Altarawneh and H. M. El-
at 273 K: 19-37),22 and fl-CTFs (selectivity of CO2/N2 at 273 K:
Kaderi, Chem. Mater., 2015, 27, 1349-1358.
,
3
†
13~35),30 but lower than that of Azo-COPs (selectivity of CO2/N2: 12. H. A. Patel, S. Hyun Je, J. Park, D. P. Chen, Y. Jung, C. T. Yavuz
73~124 at 273 K, 113~142 at 298 K) containing azo groups.12
and A. Coskun, Nat. Commun., 2013,
13. J. Lu and J. Zhang, J. Mater. Chem. A, 2014,
14. L. Tao, F. Niu, D. Zhang, J. Liu, T. Wang and Q. Wang, RSC
Adv., 2015, , 96871-96878.
4, 1357.
2
, 13831-13834.
Conclusions
5
15. X. Zhu, C.-L. Do-Thanh, C. R. Murdock, K. M. Nelson, C. Tian,
S. Brown, S. M. Mahurin, D. M. Jenkins, J. Hu, B. Zhao, H. Liu
In conclusion,
a
novel dicyano monomer containing
benzimidazole groups (DCBI) was designed and synthezed by
an oxidative coupling catalyzed by CAN at room temperature.
A series of CTFs containing benzimidazoles (CTF-BIs) were
prepared by the dynamic trimerization under ionothermal
condition. The CTF-BIs exhibited BET surface areas as high as
up to 1549 m2 g-1. The capacity of adsorption of CO2 for all the
CTF-BIs was studied and the CO2 uptake capacity was as high
as up to 21.68wt% at 273 K and 1.10 bar for CTF-BI-11. Some
CTF-BIs exhibited high selectivity of CO2 over N2, which made
these materials potential candidates for applications in CO2
capture and storage technology.
and S. Dai, ACS Macro Lett., 2013,
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18. M. Zhang, Z. Perry, J. Park and H.-C. Zhou, Polymer, 2014, 55
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19. A. K. Sekizkardes, S. Altarawneh, Z. Kahveci, T. İslamoğlu and
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The authors thank National Natural Science Foundation of
China (No. 51103168, 21201093 and 51403219) for financial
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